In MySQL there are tools like mysqltuner.pl and tuning-primer.sh, that can help you optimize the performance of your databases.
Is there something similar for Postgresql?
As far as I know there are no good scripts to automatically tune Postgresql. The defaults are pretty sane for a small installation, and it's hard to write a script that does much better guesses for large scale databases, as it is so dependant on your application.
Instead of relying on a script I would strongly recommend that you read up a bit on Postgresql tuning. Start with:
Beyond the basic settings, my experience is that the most commonly overlooked factor is indexes and analysis to ensure the query planner have good data to work with.